An X-ray detector of the type for which the present invention is particularly adapted is shown in U.S. Pat. Nos. 4,394,578 and 4,570,071. These detectors generally comprise a housing surrounding a chamber with a radiation window aligned on one side of the chamber and a slot formed in another side of the chamber for admitting a circuit board into the chamber. The circuit board supports an array of elongated charge detector elements on one surface. An electrically conductive plate is positioned within the chamber and arranged substantially parallel with the array of detector elements. The slot is sealed about the detector board so that the chamber is gas impervious. An ionizable gas is contained in the chamber under very high pressure to provide ions when excited by radiation entering through the window. A collimator is positioned outside the chamber and oriented to pass a thin beam of radiation through the window into the chamber. The detector measures the amount of received radiation by monitoring the charge transferred between the electrically conductive plate and the detector elements from ions created in the gas as it is excited by the radiation.
Radiation detectors of the type described above are used in various applications. Commonly, such detectors are used in X-ray inspection techniques for factory applications. An example of the use of such a detector could be in the X-ray inspection of turbine blades for aircraft engines. The effectiveness of such X-ray inspection systems is directly dependent upon the efficiency of the detector. One of the features of such detectors which affects its efficiency is the need to provide a relatively thick X-ray radiation transmissive window in order to support the high pressure gas contained within the chamber. For example, the gas within the radiation chamber may be at a pressure of about 1100 pounds per square inch. Typically, the radiation window is formed by milling a portion out of an aluminum block to form one side of the pressure chamber. This block is then attached to another block in which a similar concavity has been milled so that when the two blocks are placed together, a pressure chamber having an approximately oval shape in cross-section is formed. The support block is normally formed of steel or other suitably strong material since it does not have to pass radiation and can be made sufficiently massive to withstand the pressure within the chamber without deformation. The block containing the window is, however, generally formed of aluminum in order to provide minimal attenuation of the radiation passing through the window. Deformation of the aluminum portion in a direction perpendicular to the direction of radiation entering the chamber can be accommodated by bolting the aluminum portion to the steel portion. However, in order to prevent the window from being deformed in a line parallel to the direction of radiation, it is necessary to leave sufficient thickness of aluminum in that window area to support the high pressure within the chamber. Typically, a minimum window thickness is approximately 1/8 of an inch. In applications where it is necessary to inspect low density materials, the attenuation ratio between the detector window and the part to be inspected becomes critical.